1. Field of the Invention
This invention relates to actuators, and more particularly, to flexible electret actuators and methods of manufacturing the same.
2. Background of the Invention
In the recent years, there have been continued developments for electronic products. One design concept for those developments has been providing lightweight, thin, portable, and/or small devices. In this regard, flexible electronic technology has been increasingly used in various applications, such as LCDs, flex circuits and flexible solar cells. Applications for flexible electronics, such as flexible speakers, may benefit from their low profile, reduced weight, and/or low manufacturing cost.
A loudspeaker may produce sound by converting electrical signals from an audio amplifier into mechanical motions. Moving-coil speakers are widely used currently, which may produce sound from the forward and backward motions of a cone that is attached to a coil of wire suspended in or movably coupled with a magnetic field. A current flowing through the coil may induce a varying magnetic field around the coil. The interaction of the two magnetic fields causes relative movements of the coil, thereby moving the cone back and forth. This compresses and decompresses the air, and thus generating sound waves. Due to structural limitations, moving-coil speakers are less likely to be made flexible or in a low profile.
An electrostatic speaker may operate on the principle of Coulomb's law that two conductors with equal and opposite charge may generate a push-pull force between them. The push-pull electrostatic force may cause vibration of a diaphragm, thereby generating sound. An electrostatic speaker may include two porous electrodes and a diaphragm placed between the electrodes to form a series of capacitors. The electrodes and the diaphragm may be separated by dielectric materials. The low-profile and lightweight diaphragm makes the electrostatic speaker superior to other types of speakers, such as dynamic, moving-coil or piezoelectric speakers, with respect to its transition response, expansion capability in high frequency, smoothness of sound, acoustic fidelity and low distortion.
With the simple structure, electrostatic speakers may be manufactured in various sizes to accommodate increasing demands for small and thin electronic devices. However, some electrostatic speakers may require a DC-DC converters for providing high voltage to the speakers. Considering the size, cost and power consumption of DC-DC converters, some electret materials have been developed to reduce or avoid the need of DC-DC converters.
FIG. 1 illustrates an exemplary electret speaker, which may include porous electrodes 110a and 110b with a number of holes 112a and 112b on each electrode having a porosity of at least 30 percent. The electrodes 110a and 10b may be made of metals or plastic materials coated with a conductive film. The holes 112a and 112b may be provided for allowing sound waves to pass through them. The electret speaker may further include a diaphragm 120, which may include a conductive layer 122 sandwiched between electret layers 124a and 124b. The electret layers 124a and 124b may store positive or negative charges. The electrodes 110a and 110b, and diaphragm 120 may be held in place by holding members 130a and 130b. Elements 140a, 140b, 142a and 142b may be made of insulating materials and may be used for separating the diaphragm 120 from the electrode plates 110a and 110b to form cavities 150a and 150b for the diaphragm 120 to vibrate.
In operating of an electret speaker of FIG. 1, each signal source 160a and 160b may output equal and opposite alternating signals to the electrodes 110a and 110b via conductive lines 162a and 162b. The signals may cause a time-varying electric field to develop between the electrodes 110a and 110b and the electret layers 124a and 124b, thus resulting in a push-pull force. The push-pull force may cause the diaphragm 120 to vibrate, resulting in sound waves that may pass through holes 112a and 112b. 